The invention relates generally to automatic frequency controls for television signals, and deals more particularly with an automatic frequency control which can track a television signal before and after a period of absence.
Heretofore, automatic frequency controls have been used to track video carriers of television signals at intermediate frequencies to optimize the reception of the television video and audio as described in more detail below. However, if the television signal is absent or interrupted even for a few milliseconds, the prior art automatic frequency control may substantially shift the intermediate frequency associated with the previous video carrier in a vain attempt to locate the video carrier. Consequently, when the video signal returns, the automatic frequency control may not be able to locate the video carrier.
The complete loss of the video signal may occur when one television segment ends and a new segment begins, or when a transmission satellite shifts between different up-links. Consequently, a manual control was required in the prior art automatic frequency control to aid the automatic frequency control in tracking the video carrier after a period of absence. The manual control is coupled to and activated by a channel selector such that when the channel is changed or a push button associated with the same channel is depressed, the video carrier is shifted to a nominal intermediate frequency which can be detected by the automatic frequency control. Even though the control can be activated by a viewer, the loss of the video signal is annoying, and the viewer may not know how to correct the problem. Also, if the loss occurs when a television program is being video-taped and no viewer is present, the remaining segment of the program may be lost.
Accordingly, a general object of the present invention is to provide an automatic frequency control which can track a television signal before and after a period of absence.
Another general object of the present invention is to provide a control of the foregoing type which does not interfere with normal tracking of the video carrier, or decrease the range of tracking.